TWI622654B - High strength steel plate - Google Patents

Abstract

The high-strength steel sheet of the present invention has a chemical composition of, in mass%, C: 0.10 to 0.24%, Mn: 3.50 to 12.00%, Si: 0.005 to 5.00%, Al: 0.005 to 5.00%, P: 0.15%. Hereinafter, S: 0.030% or less, N: 0.020% or less, O: 0.010% or less, Cr: 0 to 5.00%, Mo: 0 to 5.00%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Nb: 0~0.50%, Ti: 0~0.50%, W: 0~0.50%, B: 0~0.010%, Ca: 0~0.05%, Mg: 0~0.05%, Zr: 0~0.05%, REM: 0 ~0.05%, Sb: 0~0.50%, Sn: 0~0.50%, As: 0~0.05%, V: 0~2.0%, the rest is Fe and impurities, metal structure at 1/4 plate thickness In terms of area, it is: residual Worthite iron: 10.0~55.0%, high temperature tempered 麻田散铁: 30.0~75.0%, low temperature tempered 麻田散铁: 15.0~60.0%, the rest is: fresh Ma Tian loose iron: 0~10.0%, Bora iron: 0~5.0%, toughened iron: 0~5.0%.

Description

High strength steel plate

The present invention relates to a high-strength steel sheet, in particular, a high-strength steel sheet excellent in formability.

In order to make the car body and parts of the car lightweight and safe, the material is the high strength of the steel plate. In general, when the steel sheet is increased in strength, the uniform elongation, the local extensibility, and the hole expandability are all lowered, and the formability is impaired. Therefore, in order to use a high-strength steel sheet as a member for an automobile, it is necessary to obtain an appropriate balance between strength (tensile strength, relief stress) and formability.

In the conventional technical solution, a so-called TRIP steel sheet which utilizes the abnormality-induced plasticity of the residual Worthite iron is used (for example, refer to Patent Documents 1 and 2). Further, for example, the technical solutions of Patent Documents 3 and 4 are for the purpose of improving the hole expandability and local extensibility of the TRIP steel sheet, and the toughening iron or the tempered granulated iron is the main metal structure.

Further, for example, the technical solution of Non-Patent Document 1 is to use steel having a content of Mn exceeding 3.0% as the residual amount of Worthite iron. More than TRIP steel, and ductility is higher than the high ductility steel plate of TRIP steel.

Further, in the technical solution disclosed in Patent Document 5, in order to improve the hole expandability, the tempering treatment is utilized. Ma Tian loose iron is harder than other tissues, and the hardness difference from the surrounding tissue is large, and local extensibility and hole expandability are deteriorated. The hole expandability can be improved by tempering the granulated iron at a low temperature of 500 ° C or lower.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] JP-A-61-217529

[Patent Document 2] Japanese Laid-Open Patent Publication No. 05-059429

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-330584

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2011-241474

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2012-237054

[Non-patent literature]

[Non-Patent Document 1] Furukawa, Matsumura: Heat Treatment No. 37, No. 4, p204 (1997)

Residual Worthite iron allows C and Mn to be concentrated in the Worthite iron, and can be obtained by stabilizing the Worthite iron even at room temperature. In particular, when a carbide precipitation suppressing element such as Si or Al is added, when the toughened iron is metamorphosed, C can be concentrated in the Worthite iron, and the Worthite iron is more stabilized.

The techniques disclosed in Patent Documents 1 and 2 are techniques developed based on the above ideas. The more the added C content, the more the residual Worthite iron can be added, and as a result, a steel sheet having a good balance between strength and uniform elongation can be obtained. However, since the steel sheet having the soft ferrite iron as the main phase and the hard structure at the same time has a large difference in hardness, it is easy to cause voids and the local extensibility cannot be improved.

Further, the steel sheet disclosed in Patent Document 3 has a ductile iron having a low ductility as a main phase. Therefore, in the case of the entire steel sheet, the uniform elongation is low, and it is impossible to produce a complicated shape of the automobile member. Its problem.

In addition, when the method disclosed in Patent Document 4 is used, it is difficult to ensure a sufficient amount of residual Worthite iron in the obtained steel sheet, and thus uniform elongation is insufficient.

In addition, the steel sheet disclosed in the non-patent document 1 is a composite structure of the soft tempered granules and the hard structure. Therefore, similarly to the steel sheets of Patent Documents 1 and 2, it is difficult to improve the hole expandability. Not only that, but because of the soft tempering Ma Tian loose iron and the Worth Iron, the stress is very low, which is the problem.

In the method described in Patent Document 5, it is difficult to increase the percentage of the granulated iron which is tempered at a low temperature. The method of Patent Document 5 simply includes an annealing step of performing annealing treatment at a temperature equal to or lower than Ac ₃ , a cooling step of cooling to room temperature, and a tempering step of performing tempering treatment. In order to increase the granulated iron in the tempering treatment at a low temperature, it is necessary to use the above cooling to increase the granulated iron.

However, in order to increase the amount of loose iron in the field, it is necessary to destabilize the Worthite iron in the annealing process. Therefore, the Worthite iron becomes unstable, so the percentage of the residual Worth iron is reduced, and even if the percentage is ensured, the stability is lowered, and there is a problem that the EL (elongation) is remarkably lowered.

Further, as described later, since the size of the residual Worth iron in the high-temperature tempered granulated iron cannot be sufficiently reduced, the high uniform elongation and local extensibility of the object of the present invention cannot be achieved, and High relief stress and high strength.

As described above, the conventional steel system is difficult to combine the product of tensile strength and local extensibility, and the product of the lodging stress and the uniform elongation.

The inventors of the present invention have proposed a technical solution for an alloyed hot-dip galvanized steel sheet having uniform deformation and local deformability in the international patent application PCT/JP2016/067448. However, the international patent application PCT/JP2016/067448, although it can combine: the product of tensile strength and local extensibility, and the product of the stress and the uniform elongation, but because of the high C content, sometimes it leads to a point. The weldability of the weld deteriorates and must be studied on the current pattern of the spot weld.

Therefore, in the present invention, it is directed to: limiting C The method of ensuring the percentage of residual Worth iron above the quantitative amount was also reviewed.

The object of the present invention is to provide a high-strength steel sheet containing residual Worthite iron (hereinafter sometimes referred to as "residual γ") and having a Mn content of 3.50% by mass or more and a C content of 0.24% by mass or less, providing: High-strength steel sheet with high consistent elongation and local extensibility.

The present invention has been developed in order to solve the above problems, and the following high-strength steel sheets are considered as the gist of the present invention.

(1) A high-strength steel sheet having a chemical composition of, in mass%, C: 0.10 to 0.24%, Mn: 3.50 to 12.00%, Si: 0.005 to 5.00%, Al: 0.005 to 5.00%, P: 0.15% Hereinafter, S: 0.030% or less, N: 0.020% or less, O: 0.010% or less, Cr: 0 to 5.00%, Mo: 0 to 5.00%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Nb: 0~0.50%, Ti: 0~0.50%, W: 0~0.50%, B: 0~0.010%, Ca: 0~0.05%, Mg: 0~0.05%, Zr: 0~0.05%, REM : 0~0.05%, Sb: 0~0.50%, Sn: 0~0.50%, As: 0~0.05%, V: 0~2.0%, the rest: Fe and impurities, at 1/4 plate thickness Metal structure, in area%, containing residual Worthite iron: 10.0~55.0%, high temperature tempered 麻田散铁: 30.0~75.0%, low temperature tempered 麻田散铁: 15.0~60.0%, the rest is fresh Ma Tiansan Iron: 0~10.0%, Bora: 0~5.0%, toughened iron: 0~5.0%.

(2) The high-strength steel sheet according to the above (1), wherein the total area ratio of the fresh granulated iron, the ferritic iron, and the toughened iron in the metal structure is 0 to 5.0% in terms of area%.

(3) The high-strength steel sheet according to the above (1) or (2), wherein the area ratio of the ferrite and the toughened iron in the metal structure is 0%.

The high-strength steel sheet according to any one of the above-mentioned (1), wherein the steel sheet has a tensile strength of 1180 MPa or more and a sheet thickness of 0.8 to 3.2 mm.

(5) The high-strength steel sheet according to any one of the above (1) to (4), wherein the chemical composition is C: 0.13 to 0.21% by mass%.

(6) The high-strength steel sheet according to any one of the above (1) to (5), wherein the chemical composition is Mn: 4.0 to 7.0% by mass%.

(7) The high-strength steel sheet according to any one of the above (1) to (6), wherein the chemical composition component is, by mass%, Cr: 0 to 1.50%.

(8) The high-strength steel sheet according to any one of the above (1) to (7), wherein the chemical composition component is Mo: 0 to 1.00% by mass%.

(9) The high-strength steel sheet according to any one of the above (1) to (8), wherein the chemical composition component is, by mass%, Ni: 0 to 1.50%.

(10) The high-strength steel sheet according to any one of the above (1) to (9), wherein the chemical composition is Cu: 0 to 1.50% by mass%.

(11) The high-strength steel sheet according to any one of the above (1) to (10), wherein the chemical composition is B: 0 to 0.003% by mass%.

(12) The high-strength steel sheet according to any one of (1) to (11), wherein the surface of the steel sheet has a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or a Zn-Ni alloy Plating layer.

According to the present invention, it is possible to obtain a high-strength steel sheet having high uniform elongation and local extensibility.

Fig. 1 is a graph showing the relationship between the area ratio of low temperature tempered granules and YS x uEL.

Fig. 2 is a graph showing the relationship between the area ratio of the low-temperature tempered Matian loose iron and YR.

Fig. 3 is a graph showing the relationship between the area ratio of the residual Worthfield iron and YS × uEL.

Fig. 4 is a graph showing the relationship between the area ratio of the residual Worthfield iron and TS × lEL.

Fig. 5 is a graph showing the relationship between the area ratio of the high-temperature tempered granulated iron and the YS × uEL.

Fig. 6 is a graph showing the relationship between the area ratio of fresh granulated iron and TS × lEL.

The present inventors have made continuous efforts to study the technical solutions for solving the above problems. As a result, a kind of originality was found, that is, a certain amount of residual γ was dispersed in the steel sheet, and the high-temperature tempered granulated iron after tempering at a high temperature and the low-temperature tempered tempering after tempering at a low temperature Iron, which is present in the steel plate in a quantitatively composite manner In this case, a higher uniform elongation (uEL) and local extensibility (lEL) can be achieved; there is also a higher stress (YS) and tensile strength (TS).

The usual TRIP steel is such that residual Worth iron is present in the steel to enhance uniform elongation, but the stress is reduced due to the presence of ferrite. The technical solution for improving YS is a method of using ductile iron or tempered granulated iron as the main phase, but because of the low ductility of the main phase, this technique cannot improve uniform elongation.

The present inventors have found a novelty in which YS can be improved by causing a high-temperature tempered granulated iron in the steel to have excellent balance between ductility and hardness, and low-temperature tempering of granulated iron. There is a residual Worth iron in the steel, which can improve YS×uEL and TS×lEL.

The present invention has been developed based on the above-mentioned novelty. Hereinafter, each requirement of the present invention will be described in detail.

(A) chemical composition

The reason for limiting each element is as follows. In addition, in the following description, "%" of content means "mass %."

C: 0.10~0.24%

C is an essential element used to increase the strength of the steel sheet and to ensure the retention of the Worthite iron. In addition, C is also an element that contributes to the strength of the low temperature tempered granitic iron. When the C content is less than 0.10%, it is difficult to obtain sufficient steel sheet strength and a sufficient amount of residual Worthite iron. On the other hand, C If the content is higher than 0.24%, the iron and the ferritic iron will be precipitated in a large amount, and the local ductility will be greatly reduced. Therefore, the C content is set to 0.10 to 0.24%. The C content is more preferably 0.12% or more or 0.13% or more; more preferably 0.15% or more or 0.17% or more. Further, the C content is more preferably 0.24% or less or 0.23% or less; more preferably 0.22% or less or 0.21% or less.

Mn: 3.50~12.00%

Mn is an essential element for ensuring the retention of Worthite iron in the same manner as C. When the Mn content is less than 3.50%, the effect of addition thereof cannot be sufficiently exerted. On the other hand, if the Mn content is higher than 12.00%, the amount of iron in the Worthfield is excessively increased, and the low-temperature tempered granulated iron is not obtained, and the tensile strength and the undulation stress are lowered. The Mn content is preferably 3.80% or more or 4.00% or more, and more preferably 4.40% or more, 4.80% or more, or 5.10% or more. Further, the Mn content is preferably 11.00% or less or 10.00% or less, more preferably 9.00% or less, 8.00% or less, or 7.00% or less.

Si: 0.005~5.00%

Al: 0.005~5.00%

Both Si and Al are deoxidizers, and are also elements which stabilize ferrite iron during annealing and have an effect of suppressing precipitation of ferritic iron. When both Si and Al are less than 0.005%, the effect of addition can not be sufficiently exerted. On the other hand, if both Si and Al are contained in an amount of more than 5.00%, surface properties, paintability, and weldability are deteriorated. Therefore, the contents of Si and Al are both set at 0.005 to 5.00%.

The content of Si and Al is preferably set to 0.010% or more, more preferably 0.020% or more, and more preferably 0.030% or more. In particular, Si may be set to 0.50% or more, 0.90% or more, or 1.05% or more. Further, the content of Si and Al is preferably set to 3.50% or less, more preferably 2.50% or less, and more preferably 2.10% or less. In particular, Al may be set to 1.00% or less.

In addition, if the Al content is higher than 5.00%, the δ ferrite iron remains at room temperature. When the δ fat iron is hot rolled, it will become an elongated ferrite iron. Further, during the tensile test and the press forming, stress is concentrated on the ferrite iron, and the test piece or the steel sheet is easily broken. For this reason, the Al content is set to 5.00% or less. If you want to improve the material of the steel plate, it is better to set Si+Al to 0.80% or more, and more preferably 1.00% or more.

P: 0.15% or less

P is an impurity element which is inevitably mixed from the steel raw material. If the P content is more than 0.15%, ductility and weldability may be deteriorated. Therefore, the P content is set to 0.15% or less. The P content is more preferably 0.10% or less, 0.05% or less, or 0.020% or less. Although the lower limit is 0%, if the P content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so the lower limit can also be set to 0.0001%.

S: 0.030% or less

S is an impurity element that is inevitably mixed from the steel raw material. If the S content is more than 0.030%, MnS which is elongated by hot rolling will be formed. Formability such as properties and hole expandability is deteriorated. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.015% or less or 0.009% or less. Although the lower limit is 0%, if the S content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so the lower limit can also be set to 0.0001%.

N: 0.020% or less

N is an impurity element that is inevitably mixed in from the steel raw material or in the steel making process. When the N content is more than 0.020%, the ductility is lowered. Therefore, the N content is set to be 0.020% or less. The N content is preferably 0.015% or less, 0.010% or less, 0.0070% or less, or 0.0050% or less. Although the lower limit is 0%, if the N content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so the lower limit can also be set to 0.0001%.

O: 0.010% or less

O is an impurity element which is inevitably left after deoxidation. If the O content is more than 0.010%, the ductility will be lowered. Therefore, the O content is set to be 0.010% or less. The O content is preferably 0.007% or less, 0.004% or less, or 0.0025% or less. Although the lower limit is 0%, if the O content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so the lower limit can be set to 0.0001%.

In the alloyed hot-dip galvanized steel sheet according to the present invention, in addition to the above elements, Cr, Mo, and the contents shown below may be further contained. One or more elements selected from Ni, Cu, Nb, Ti, W, B, Ca, Mg, Zr, REM, Sb, Sn, As, and V.

Cr: 0~5.00%

Mo: 0~5.00%

Ni: 0~5.00%

Cu: 0~5.00%

Cr, Mo, Ni, and Cu are all elements that contribute to the strength of the steel sheet, and therefore may be included as necessary. However, if any of Cr, Mo, Ni, and Cu is more than 5.00%, the strength is too high and the ductility is lowered. Therefore, the contents of Cr, Mo, Ni, and Cu are all set to 5.00% or less.

The content of any one of Cr, Mo, Ni, and Cu is preferably 4.00% or less or 3.00%, more preferably 2.00% or less, or 1.00% or less, more preferably 0.80% or less or 0.50% or less. Further, although the lower limit of these elements is 0%, in order to obtain the above effects, the content of one or more selected from the above elements may be 0.01% or more, or may be 0.02% or more. However, in order to reduce the cost of the alloy, the total content of these elements may be 2.00% or less, 1.50% or less, 1.10% or less, 0.7% or less, or 0.40% or less.

Nb: 0~0.50%

Ti: 0~0.50%

W: 0~0.50%

Nb, Ti, and W are elements that can form fine carbides, nitrides, or carbonitrides and are useful for improving the strength of the steel sheet, and therefore may be included as necessary. However, if the content of any of Nb, Ti, and W is more than 0.50%, the strength is too high and the ductility is lowered. Therefore, the contents of Nb, Ti, and W are all set to 0.50% or less.

The content of any one of Nb, Ti and W is preferably 0.40% or less or 0.20% or less, more preferably 0.10% or less or 0.05% or less. Further, although the lower limit of these elements is 0%, in order to obtain the above-described effects, the content of one or more selected from the above elements may be set to 0.005% or more, or may be set to 0.008% or more. However, in order to reduce the cost of the alloy, the total content of these elements may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less.

B: 0~0.010%

B is a kind of delaying metamorphism, which is helpful for improving the strength of the steel sheet, and is an element that segregates at the grain boundary and contributes to the strengthening of the grain boundary, and therefore may be contained as necessary. However, if the B content is more than 0.010%, the B compound will precipitate in a large amount and thus cause embrittlement of the steel sheet. Therefore, the B content is set to be 0.010% or less. The B content is preferably 0.005% or less or 0.0030% or less, more preferably 0.0020% or less or 0.0016% or less. Further, although the lower limit of B is 0%, in order to obtain the above effects, the B content may be set to 0.0002% or more, or may be set to 0.0003% or more.

Ca: 0~0.05%

Mg: 0~0.05%

Zr: 0~0.05%

REM: 0~0.05%

Ca, Mg, Zr, and REM (rare earth elements) are elements that can control the shape of sulfides and oxides and are useful for improving local ductility and hole expandability, and therefore may be contained as necessary. However, when the content of any of Ca, Mg, Zr, and REM is more than 0.05%, the workability is deteriorated. Therefore, the contents of Ca, Mg, Zr, and REM are all set to 0.05% or less.

The content of any one of Ca, Mg, Zr and REM is preferably 0.03% or less or 0.01% or less, more preferably 0.0060% or less or 0.0040% or less. In addition, if you want to select two or more of Ca, Mg, Zr, and REM for compound addition, it is preferable to set the total content to 0.05% or less or 0.02% or less, preferably 0.01% or less or 0.0060% or less. . Further, although the lower limit of these elements is 0%, in order to obtain the above effects, the content of one or more selected from the above elements may be set to 0.0001% or more, or may be set to 0.0002% or more.

Here, in the present invention, REM (rare earth element) means a total of 17 elements of Sc, Y, and lanthanoid elements, and the content of the above REM means the total content of these elements. Further, the lanthanoid element is industrially added in the form of a bismuth alloy.

Sb: 0~0.50%

Sn: 0~0.50%

Sb and Sn can suppress the formation of oxides by oxidizing elements such as Mn, Si, and/or Al in the steel sheet, which form an oxide, and have an effect of improving surface properties and plating properties. And contain. However, if the content of any one of Sb and Sn is higher than 0.50%, the effect of addition tends to be saturated. Therefore, the contents of Sb and Sn are all set to 0.50% or less.

The content of any one of Sb and Sn is preferably 0.35% or less or 0.15% or less, more preferably 0.08% or less or 0.03% or less. Further, although the lower limit of these elements is 0%, in order to obtain the above effects, one or more kinds selected from the above elements may be set to 0.010% or more.

As: 0~0.05%

In the same manner as Sb and Sn, it is possible to suppress the formation of oxides by oxidizing elements such as Mn, Si, and/or Al in the steel sheet to form an oxide, thereby improving surface properties and plating properties. Elements, so they can be included as needed. However, when the As content is more than 0.05%, the addition effect tends to be saturated. Therefore, the As content is set to 0.05% or less. The As content is preferably 0.02% or less, more preferably 0.01% or less. Further, although the lower limit of As is 0%, in order to obtain the above effect, the As content may be set to 0.005% or more. The total content of Sb, Sn, and As may be set to 0.05% or less, 0.03% or less, or 0.01% or less, as necessary.

V: 0~2.0%

V is an element which forms precipitates and refines crystal grains, and is useful for improving strength and toughness, and therefore may be contained as necessary. However, if the V content is more than 0.50%, the above effects tend to be saturated, and the manufacturing cost will rise. Therefore, the V content is set to 2.0% or less or 1.0% or less. The V content is preferably 0.50% or less or 0.30%% or less, more preferably 0.10% or less, and even more preferably 0.06% or less. Further, although the lower limit of V is 0%, in order to obtain the above effects, the V content may be set to 0.001% or more or 0.005% or more.

In the chemical composition of the steel sheet of the present invention, the remainder is Fe and impurities.

The term "impurity" as used herein means a component which is mixed in a raw material such as ore, recycled waste, or mixed in various steps in the manufacturing process when the steel sheet is produced on an industrial scale, and is not incorporated in the present invention. The range of adverse effects that can be tolerated.

(B) Metal structure of the steel plate

Here, the metal structure of the steel sheet of the present invention at a position of 1/4 plate thickness will be described. The area ratio of each tissue changes depending on the annealing conditions, and it affects mechanical properties such as strength, uniform elongation, and local extensibility. The desired mechanical properties, for example, will vary due to automotive parts, and annealing conditions may be selected to control the area ratio of each tissue as necessary. In addition, "%" in the following description means "area%".

Residual Worth Iron: 10.0~55.0%

Residual Vostian iron (hereinafter also referred to as "residual gamma") is an organization that can contribute to the improvement of ductility, especially the promotion of uniform elongation, by metamorphosis-induced plasticity. In the steel sheet of the present invention, in order to obtain excellent uniform elongation, it is necessary to set the area ratio of residual γ to 10.0% or more. On the other hand, when the area ratio of the residual γ is more than 55.0%, the lodging stress is lowered, so that the area ratio of the residual γ is set to 55.0% or less. The area ratio of the residual γ is preferably 13.0% or more, 15.0% or more, or 18.0% or more, more preferably 20.0% or more. Further, the area ratio of the residual γ is preferably 50.0% or less, and more preferably 45.0% or less, 40.0% or less, 35.0% or less, or 31.0% or less.

The residual γ of the metal structure in the present invention is mainly composed of a residual γ in an elongated form such as a mesh of granulated iron. The thickness is below 1 μm, and most of it is about 0.02 to 0.1 μm. When such a mesh-like residual γ is coexisted with a low-temperature tempered granulated iron which will be described later, it is difficult to achieve by a conventional method, and therefore, it can be achieved by using a production method to be described later. In addition, the conventional method is to form a single phase of the Vostian iron, then cool it to room temperature, and after it becomes a granulated iron, it is heated in the range of the two-phase state, so that the C and Mn are concentrated. A method of obtaining residual γ in the sita iron (for example, refer to Non-Patent Document 1 and Patent Document 4).

In the present invention, as described later, it is necessary to form a state in which the Worthite iron and the granulated iron coexist in the second cooling step. Then, using the second annealing process to generate Worthian from the granulated iron Iron, which turns the Worthite iron into a grid-like elongated structure. The organization around the Worthfield Iron is a high temperature tempered granulated iron. This Vostian iron will become a residual Worthfield iron during cooling to room temperature.

The Worthite iron after the second cooling step is a relatively coarse structure, and becomes a granulated iron by the third cooling process. By the above process, the grid-shaped Worth iron can be coexisted with the low-temperature tempered granulated iron.

High temperature tempering Ma Tian loose iron: 30.0~75.0%

High-temperature tempering Ma Tian loose iron, mainly using the temperature of 550 ~ 700 ° C to tempering after the Ma Tian loose iron, the determination method is described in detail. In order to improve the formability, the area ratio of the high-temperature tempered Ma Tian loose iron is set at 30.0 to 75.0%. The area ratio of the high-temperature tempered granulated iron is preferably 33.0% or more, 36.0% or more, or 38.0% or more, and is preferably 70.0% or less, 65.0% or less, 60.0% or less, or 55.0% or less.

Low temperature tempering Ma Tian loose iron: 15.0~60.0%

The low-temperature tempered granulated iron is mainly composed of fresh granulated iron which is produced by the third cooling step described later, and is tempered at a temperature of 250 to 480 ° C. The measurement method is detailed later. Description.

The low-temperature tempering of the granulated iron in the field is relatively uniform, but it is less likely to lower the local extensibility, and the lodging stress and the tensile strength are superior to the fresh ramie iron which will be described later. Therefore, the area ratio of the low-temperature tempered granita iron is set at 15.0% or more. Low temperature tempering The area ratio may be set in accordance with the desired intensity level. However, if too much, the uniform elongation is lowered. Therefore, it is set to 60.0% or less. In order to improve the formability, the lower limit of the low temperature tempered 麻田散铁 may also be set at 20.0%, 25.0%, 30.0%, 34.0% or 38.0%. In order to improve uniform elongation, the upper limit can also be set at 55.0%, 50.0%, 46.0% or 42.0%.

The steel sheet of the present invention is in the metal structure at the position of 1/4 of the thickness of the sheet, and the rest is fresh granulated iron, bun iron, and toughened iron.

Fresh Ma Tian loose iron: 0~10.0%

In the tempering process, a little snowy iron is precipitated from the Worthite iron, and the Worthite iron becomes unstable, and the fresh ramie loose iron can be produced by the cooling process after the tempering process. When the area ratio of the fresh yam iron is more than 10.0%, not only the YS and the local extensibility are lowered, but the area ratio of the residual γ is also reduced, and the uniform elongation is also lowered. Therefore, the area ratio of the fresh Ma Tian loose iron is set at 10.0% or less. The area ratio of fresh granulated iron is set to be less than 5.0%, preferably 3.0% or less, and 0%, in other words, the best tissue without fresh granulated iron.

Bora: 0~5.0%

In the middle of cooling during annealing or in the alloying treatment of the plating layer, it is sometimes generated from the Worthite iron. When the area ratio of the Boron iron exceeds 5.0%, the area ratio of the residual γ is reduced, and the strength and ductility are drastically reduced. Therefore, the area ratio of the Bora iron is set at 5.0%. under. The area ratio of the Borne iron is as low as possible, and it is preferably set at 3.0% or less, and 0% is best.

Toughened iron: 0~5.0%

The metal structure of the present invention may also contain toughened iron. According to the Mn content in the steel sheet of the present invention, the metamorphic iron is not easily deformed, and therefore the area ratio of the toughened iron is controlled to 5.0% or less. The area ratio of the toughened iron is preferably 3.0% or less, and 0% is the best.

In addition, the total area ratio of the fresh Ma Tian loose iron, the Bora iron, and the toughened iron may be controlled to 5.0% or less, 3.0% or less, or 1.0% or less. It is more preferable to control the total area ratio of the metal parts of these remaining parts to 0%.

Hereinafter, a method for obtaining the area ratio of each of the above-described tissues in the present invention will be described.

First, a section parallel to the rolling direction is cut, mirror-polished, and then electrolytically ground, and then the sample is subjected to SEM-EBSD for a position at a quarter of the thickness of the sheet from the surface ( Hereinafter, the field of 100 μm × 100 μm or more, which is referred to as "plate thickness 1/4 position"), is measured at intervals of 0.1 μm. Then, using the analysis software produced by TSL Solutions, the average of the image quality (Grain Average Image Quality: GAIQ value) in the grains of each crystal grain was calculated. Then, the area ratio of the area judged to be the FCC is regarded as the area ratio of the remaining Worth Iron.

Next, according to the method described in Fse Lepera: Journal of Metals 32, No. 3, (1980) 38-39, the section parallel to the rolling direction is etched to show the fresh ramie loose iron and residual wa Sita Iron. Then, at a position of 1/4 of the thickness of the plate, an optical microscope was used to observe at a magnification of 1000 times, and the photographed tissue photograph was image-processed to determine the total area of the fresh granulated iron and the residual Worth iron. rate(%). The etching solution used at this time is a solution in which 4% of Na ₂ S ₂ O ₅ is dissolved in distilled water, and a solution in which 4% of [C ₆ H ₂ (NO) ₃ OH] is dissolved in ethanol. Corrosion fluid mixed together at a ratio of 1:1.

Then, the area ratio of the residual Worthite iron measured by the above method was subtracted from the value of the total area ratio of the fresh mashed iron and the residual Worth iron, and the area ratio of the fresh granulated iron was obtained.

Then, the percentage of crystal grains having a GAIQ value of 5,000 or less at a position of 1/4 of the plate thickness is used as a total area ratio of low temperature tempered granian iron and fresh granulated iron. Then, the area ratio of the fresh granulated iron is subtracted from this value, and the area ratio of the low-temperature tempered granulated iron is determined.

Further, a cross section perpendicular to the rolling direction was cut, and after mirror polishing, corrosion was performed with a nitric acid etching solution. The sample was observed by SEM. When the SEM was used for observation, the field of measurement was performed at a magnification of 5000 times, and the field of view of 4 μm or more was observed for a field of 25 μm × 20 μm at a position of 1/4 of the plate thickness. After etching with a nitric acid etching solution, observation was carried out by SEM, and the hollowed-out tissue having no lower structure was regarded as ferrite iron or high-temperature tempered granulated iron. Among them, if the ratio of the long axis to the short axis is 2 or more, it is regarded as high temperature back. Fire Ma Tian scattered iron. The long axis and the short axis are obtained in the following manner. First, in the photograph taken as described above, among the lines connecting the grain boundary to the other grain boundary, the longest line is regarded as the long axis. Then, among the lines connecting the grain boundary of the long axis into two halves and the other grain boundary, the shortest distance is regarded as the short axis. In other words, the percentage of the tissue having a ratio of the major axis to the minor axis of 2 or more among the tissues which have been hollowed out without the lower structure is regarded as the area ratio of the high-temperature tempering and the loose iron, and the long axis and the short axis are The percentage of the structure in which the ratio of the shaft is less than 2 is regarded as the area ratio of the ferrite iron.

As for the Borne iron, after etching with a nitric acid etching solution, the area of 25 μm × 20 μm is observed by SEM at a position of 1/4 of the plate thickness to reach four observation fields, and the percentage of the layered structure that can be seen is regarded as a wave. The area ratio of iron. In addition, as for the toughened iron, after etching with a nitric acid etching solution, the area of 25 μm × 20 μm is observed by SEM at a position of 1/4 of the plate thickness, and the ratio of the long axis to the short axis is obtained. It is 2 or more, and the percentage of the structure of the ferritic iron can be confirmed by 5,000 times of SEM, and it is regarded as the area ratio of the toughened iron.

(C) Mechanical properties

The tensile strength (TS) is as high as possible, and is set to be 1180 MPa or more. For example, when the steel sheet is used as a material for automobiles, the thickness of the steel sheet can be reduced by increasing the strength, and the weight of the automobile can be reduced. The lower limit of the tensile strength can also be set to 250 MPa. The upper limit of the tensile strength is not particularly limited, but is set to be 1650 MPa or less or 1600 MPa or less. It is appropriate. In order to obtain the above-described weight reduction effect, the higher the stress (YS) of the steel sheet and the work hardening amount after the processing (after the fall), the better. The hardness due to the deformation is increased as the stress (YS) is higher and the amount of work hardening is increased.

The amount of work hardening, although the value of n can be expressed as an index, the value of n is a value similar to uEL. Therefore, in the steel sheet of the present invention, the stress (YS) x uniform elongation (uEL) is used as an index. In the steel sheet of the present invention, it is set to YS × uEL ≧ 10000 MPa%. The tensile test piece here is a test piece No. 5 using Japanese Industrial Standard JIS Z2241 (a plate-shaped test piece having a width of a parallel portion of 25 mm and a distance between the original punctuation points of 50 mm).

In order to press-form a steel sheet, it is desirable to have excellent uniform elongation (uEL) and local elongation (lEL). When the steel sheet is about to undergo local deformation, the strength of the steel sheet has reached the tensile strength (TS). Therefore, tensile strength (TS) × local elongation (lEL) is used as an index indicating such a state. In the steel sheet of the present invention, it is set to TS × 1 EL ≧ 6000 MPa%.

The lodging stress is an index for ensuring the hardness of the steel sheet after forming, and therefore, the higher the drop ratio (the stress/tensile strength), the better. The lodging ratio is preferably above 0.70. More preferably, it is 0.71 or more or 0.72 or more.

Further, in the present invention, the tensile strength and the undulation stress are values obtained by a tensile test in a direction perpendicular to the rolling direction. The direction perpendicular to the rolling direction means a direction perpendicular to the rolling direction and the thickness direction of the steel sheet, that is, the width direction direction. meaning.

(D) Manufacturing method

The steel sheet of the present invention can be produced, for example, by the production method shown below. In the following production method, the following steps (a) to (m) are sequentially performed. Therefore, each step will be described in detail.

(a) Melting process

A steel block or billet having the above chemical composition is melted. The conditions of the melting step are not particularly limited, and a general method can be employed.

(b) Hot rolling process

After the steel block or the billet is heated, hot rolling is performed to form a hot rolled steel sheet. The heating temperature before hot rolling is set at 1100 to 1170 ° C, and the refining rolling temperature for hot rolling is preferably set at 880 to 970 ° C. From the last pass (PASS) of the hot rolling to the sixth pass, it is preferable that each pass is a roll having a large rolling reduction of at least 3 times or more of 10% or more.

When the heating temperature is lower than 1,100 ° C, the temperature is lowered before the billet is sent to the hot rolling, and there is a concern that the refining rolling cannot be completed in accordance with the required temperature. On the other hand, when the heating temperature is higher than 1170 ° C, the Worthite iron during heating becomes coarser, and the crystal of the steel sheet after rolling may become coarse. Therefore, the heating temperature is set to 1170 ° C or less. should.

Further, the steel having the chemical composition according to the present invention is very hard. Therefore, if the refining rolling temperature is lower than 880 ° C, it will bring a large load to the rolling mill, and it may be difficult to carry out hot rolling. . On the other hand, when the refining rolling temperature is higher than 970 ° C, the crystal of the steel sheet after rolling may become too coarse.

(c) 1st cooling process

The hot-rolled steel sheet after the completion of the refining rolling is cooled. The cooling conditions in the first cooling step are not particularly limited, and are preferably cooled at an average cooling rate of 20 ° C /sec or more, and cooling is preferably stopped in a temperature range of 550 to 650 ° C. If it is the above range, it is easier to comply with the temperature range at the time of the winding process.

(d) Rolling process

The hot-rolled steel sheet after cooling stop is taken up. The coiling temperature is preferably 450~600 °C. When the coiling temperature is lower than 450 ° C, the shape of the sheet becomes poor. Further, in the steel sheet having a high Mn content according to the present invention, when the coiling temperature is higher than 600 ° C, the scale is thick, and pickling is not easy.

(e) Cold rolling process

The coiled hot-rolled steel sheet was again flattened, pickled, and then cold-rolled to obtain a cold-rolled steel sheet. The rolling reduction ratio is preferably set at 40 to 65%. If the rolling reduction is less than 40%, the thickness of the sheet becomes too thick. Therefore, when it is made into a car part, the weight tends to become too heavy. On the other hand, the rolling reduction rate Above 65%, it is difficult to finish cold rolling in a short time. In addition, the ductility after annealing is also lowered. The thickness of the cold rolled steel sheet is preferably in the range of 0.8 to 3.0 mm.

(f) First annealing process

After the cold rolling step, the cold rolled steel sheet is annealed at a temperature of 850 to 970 ° C for 90 seconds or more. The metal structure can be transformed into a single phase structure of Worthite iron by retention in the above temperature range. When the annealing temperature is lower than 850 ° C, or the holding time is less than 90 seconds, the amount of Worth iron is too small. In the end, the required amount of low-temperature tempered granulated iron is not ensured, and the stress is lowered.

When the annealing temperature is higher than 970 ° C, there is a concern that the furnace may be damaged. A large amount of scale is generated on the surface of the steel sheet, and after pickling, uneven marks are left on the surface of the steel sheet, which is not suitable as a steel sheet for automobiles. In addition, when the holding time is longer than 180 seconds, a large amount of scale is generated on the surface of the steel sheet, and after pickling, uneven marks are left on the surface of the steel sheet, which is not suitable as a steel sheet for automobiles. Therefore, the holding time in the first annealing step is preferably set to 180 seconds or less.

(g) second cooling process

After the first annealing step, the steel sheet is cooled to a temperature range of 150 to 250 °C. In this temperature range, metal phase metamorphism is less likely to occur. The average cooling rate is preferably 1 to 100 ° C / sec. The second cooling step is used to generate the granulated iron, which is also used by the second annealing worker described later. In order, it will become a high-temperature tempering Ma Tian loose iron and the Vastian iron after the inverter state. The area ratio of the high temperature tempered granulated iron can be adjusted by appropriately selecting the cooling temperature in accordance with the desired strength or characteristics.

In addition, in carrying out this cooling process, it is most important to coexist the Worthite iron and the granulated iron. In the second cooling step to the third cooling step, most of the Worth iron is turned into a granulated iron, and the tempering step is used to become a low-temperature tempered granulated iron. On the other hand, in the cooling step, one part of the granulated iron is a high-temperature tempered granita iron by the second annealing step as described above. Therefore, the amount of iron and the amount of iron in the field can be adjusted according to the temperature of the cooling process, whereby the amount of low-temperature tempered granulated iron in the final structure and the amount of high-temperature tempered granulated iron can be controlled to fall into the present invention. In the range.

When the cooling stop temperature is lower than 150 ° C, the Worstian iron obtained in the cooling step is reduced, so that the low-temperature tempering in the final structure of the field is less. On the other hand, when the cooling temperature is higher than 250 ° C, the amount of granulated iron is small, and finally, it is difficult to ensure that the high-temperature tempered granulated iron reaches 30.0% or more. The cooling stop temperature is preferably 180 ° C or higher, preferably 230 ° C or lower, and more preferably 220 ° C or lower.

(h) second annealing process

After the second cooling step, the steel sheet is annealed at a temperature of 550 ° C or higher and lower than the Ac1 point for 120 seconds or longer. When the annealing temperature is lower than 550 ° C, the ferritic iron and the ferritic iron will be precipitated in a large amount, and the residual Worth iron will be reduced. The annealing temperature is preferably higher than 580 ° C or higher.

On the other hand, when the annealing temperature is higher than or equal to the Ac ₁ point, the amount of residual Worstian iron obtained is small. The reason is presumed as follows. It is considered to be: Because the inverter state of the return to the Worthfield iron is too excessive, the Worthite iron when heated is too much. As a result, the C and Mn contents in the Worthite iron become too small, and the Worthite iron tends to be unstable. Then, in the second cooling step, it becomes a granulated iron, and the remaining Worth iron is reduced.

In addition, the Ac ₁ point can be calculated by the following formula.

Ac ₁ =723+29.1×Si-10.7×Mn+16.9×Cr-16.9×Ni

The symbol of each element in the above formula means the meaning of the content (% by mass) of each element.

The holding time at the above annealing temperature is set to 120 seconds or more. If the holding time is less than 120 seconds, the inverter state of the Worthfield Iron will not be returned, and the remaining Worth Iron will be reduced. The holding time can be appropriately set according to the relationship between the annealing temperature and the annealing temperature, but even if the annealing is performed for more than 8 hours, there is not much change, and in terms of industrial productivity, only the cost is obtained. It is higher, so the upper limit is set at around 8 hours.

In the second annealing step, the furnace may be placed in a furnace that has been previously heated to perform heating, or may be heated by electromagnetic induction (IH) or the like. When the heating rate is lower than 10 ° C / sec, the amount of iron in the remaining Worth is reduced. The reason for this is that it is presumed that during the heating, the ferritic iron will be precipitated in a large amount, and even if it is heated later, some of it will not be completely melted, and as a result, the C in the residual Worthite iron will be reduced. reason. On the other hand, in order to control the temperature of the second annealing step, it is essential The upper limit of the heating rate is set at about 25 ° C / sec.

(i) third cooling process

After the second annealing step, the steel sheet was cooled to room temperature. If the steel sheet is not cooled to room temperature, fresh 麻田散铁 will be formed in the tempering step described later, which may cause a decrease in YS. The average cooling rate is preferably set at 8 ° C / sec or more. When the average cooling rate is lower than 8 ° C / sec, the toughened iron is easily precipitated, and both YR and YS × uEL become low.

(j) tempering process

After the third cooling step, the steel sheet is subjected to a tempering treatment for 1 second or longer in a temperature range of 250 to 480 °C. The tempering process is used to generate low temperature tempered granulated iron. If the tempering temperature is lower than 250 ° C, sufficient tempering effect will not be obtained, and many fresh rammed iron will remain. As a result, YS will decrease, TS will become higher, and the ratio of decrease will decrease. The tempering temperature is preferably set at 200 ° C or higher.

On the other hand, if the tempering temperature is higher than 480 ° C, the low temperature tempered 麻田散铁 becomes too soft, YS and TS will be extremely reduced, and the residual Worth iron will be transformed into a wave of iron, uEL also It will decrease, so the temperature range of the tempering process is set below 480 °C. More preferably, it is below 460 ° C or 400 ° C. In addition, if the holding time exceeds one hour, the remaining Worthite iron will be reduced, so the holding time is preferably set to be less than one hour.

When the amount of (Si+Al) of the steel sheet is 0.8% by mass or more, After tempering, TS×uEL will be improved. Although the reason is not very clear, it is presumed that the C in the loose iron of the Ma Tian cannot be decomposed into the snow-light iron, so it is concentrated in the residual Worthite iron. The amount of (Si + Al) is preferably 1.0% by mass or more.

(k) 4th cooling process

After the tempering process, the steel sheet was cooled to room temperature. The cooling rate in the fourth cooling step is not particularly limited as long as it is a cooling rate equal to or higher than air cooling, and the change in the metal structure is small. However, if the cooling rate is lower than 5 ° C / sec, the amount of toughened iron may increase as in the third cooling step. On the other hand, when cooling is performed at a cooling rate of more than 80 ° C / sec, uneven cooling is likely to occur, and the shape of the plate is deteriorated. Therefore, the cooling rate is preferably set to 5 to 80 ° C / sec or less.

(1) Plating process

After the tempering step, the steel sheet cooled to room temperature by the fourth cooling step may be subjected to hot-dip galvanizing, alloying hot-dip galvanizing, or Zn-Ni alloy plating. The Zn-Ni alloy plating is carried out by electroplating. The hot-dip galvanizing is a steel sheet which is cooled to room temperature by the fourth cooling step, and is immersed in a hot-dip galvanizing bath at 460 °C. Alternatively, after the third cooling step, the steel sheet may be immersed in the hot-dip galvanizing bath, and in the tempering step, the alloying treatment of the plating layer may be performed.

(m) alloying process

The alloyed hot-dip galvanizing is performed by heating the steel sheet which has been subjected to hot-dip galvanizing to 480 to 500 ° C for alloying treatment. Similarly to the case of the hot-dip galvanized steel sheet, the alloying treatment of the plating layer may be performed in the tempering step.

Further, the plate thickness of the steel sheet to which the present invention is applied is mainly 0.8 to 3.0 mm. The upper limit of the thickness of the plate may be set to 2.8 mm or 2.5 mm as necessary.

Hereinafter, the present invention will be more specifically described in accordance with the examples, but the invention is not limited to these examples.

[Examples]

First, a billet having a thickness of 240 mm having the chemical composition shown in Table 1 was produced. This billet was hot rolled according to the conditions shown in Tables 2 and 3 to obtain a hot rolled steel sheet. At this time, in one pass (PASS), at least three times or more of rolling with a large reduction ratio of 10% or more is performed. The hot-rolled steel sheet was cooled to a coiling temperature by a spray mist, and then taken up. The hot-rolled steel sheets produced were pickled to remove scales, and then cold-rolled according to the conditions shown in Tables 2 and 3 to prepare cold-rolled steel sheets having a thickness of 1.2 mm.

[Table 1]

[Table 2]

[table 3]

A test material was taken from the obtained cold-rolled steel sheet, the test material was heated to the highest annealing temperature shown in Table 2 and Table 3, and annealing treatment according to the retention times shown in Table 2 and Table 3 was performed (first annealing) In the next step, the cooling is performed until the cooling stop temperature according to the average cooling rate shown in Tables 2 and 3 (second cooling step).

The second annealing step after the second cooling step is performed at the average heating rate shown in Tables 2 and 3, and is heated to the highest annealing temperatures shown in Tables 2 and 3, and is carried out according to Tables 2 and 3. Keep the annealing time. Next, cooling to room temperature was performed at the average cooling rate shown in Table 2 and Table 3 (third cooling step).

The tempering step was carried out by heating to the temperatures shown in Tables 2 and 3 at an average heating rate of 5 ° C / sec, and the holding times shown in Table 2 and Table 3 were maintained. Then, it was cooled to room temperature at a cooling rate of 10 ° C / sec (fourth cooling step).

For Test Nos. 57 to 59, the surface was plated. In Test No. 57, after the tempering step was completed, the Zn-Ni layer was adhered to the surface by electroplating. In Test No. 58, after the third cooling step, the steel sheet was immersed in a hot-dip galvanizing bath of Zn which had been heated to 460 ° C to form a hot-dip galvanized steel sheet. In the above-described hot-dip galvanizing bath, Al is contained in an amount of about 0.01% in the same manner as the conventional method. Instead of performing the tempering process, the temperature of the hot-dip galvanizing bath is used. Further, in Test No. 59, similarly to Test No. 58, after the third cooling step, the steel sheet was immersed in a hot-dip galvanizing bath, and then Zn and the base material Fe were subjected to reheating and maintaining the temperature. Alloying. In addition, the timing of plating is not limited to the above Process. For example, in the third cooling step, the steel sheet may be immersed in a hot-dip galvanizing bath or alloyed.

For each steel sheet manufactured by the above-described steps, the metal structure was classified by the following method. The method for determining the area ratio of each tissue is explained below.

First, a cross section perpendicular to the rolling direction is cut, and after mirror polishing, electrolytic polishing is performed, and then the sample is subjected to SEM-EBSD for an area of 100 μm × 100 μm or more at intervals of 0.1 μm. Determination. Then, using the analysis software produced by TSL Solutions, the average of the image quality (Grain Average Image Quality: GAIQ value) in the grains of each crystal grain was calculated. Then, the area ratio of the area judged to be FCC at the position of the plate thickness of 1/4 is regarded as the area ratio of the residual γ.

Next, according to the method described in FS Lepera: Journal of Metals 32, No. 3, (1980) 38-39, the cross section in the rolling direction is etched to show fresh granulated iron and residual γ. Then, at a position of 1/4 of the thickness of the plate, an optical microscope was used to observe at a magnification of 1000 times, and the photographed tissue photograph was image-processed to determine the total area of the fresh granulated iron and the residual Worth iron. rate(%). The etching solution used at this time is a solution in which 4% of Na ₂ S ₂ O ₅ is dissolved in distilled water, and a solution in which 4% of [C ₆ H ₂ (NO) ₃ OH] is dissolved in ethanol. Corrosion fluid mixed together at a ratio of 1:1.

Then, subtract the residual measured by the above method from the value of the total area ratio of the fresh Ma Tian loose iron and the residual Worth iron The area ratio of the Worthfield iron, thus the area ratio of the fresh granulated iron.

Then, the percentage of crystal grains having a GAIQ value of 5,000 or less (at a position of 1/4 of the plate thickness) is taken as the total area ratio of the low-temperature tempered granian iron and the fresh granulated iron. Then, the area ratio of the fresh granulated iron is subtracted from this value, and the area ratio of the low-temperature tempered granulated iron is determined.

Further, a cross section perpendicular to the rolling direction was cut, mirror-polished, and then etched with a nitric acid etching solution, and observed by SEM at a position of 1/4 of the sheet thickness. When observed by SEM, a magnification of 5000 times was used. The field of measurement was performed for four fields or more in the field of 25 μm × 20 μm. After etching with a nitric acid etching solution, observation was carried out by SEM, and the hollowed-out tissue having no lower structure was regarded as ferrite iron or high-temperature tempered granulated iron. Among them, the percentage of the ratio of the major axis to the minor axis of 2 or more is regarded as the area ratio of the high-temperature tempered hemp iron, and the percentage of the ratio lower than 2 is regarded as the area ratio of the ferrite iron. The long axis and the short axis are obtained in the following manner. First, in the photograph taken as described above, among the lines connecting the grain boundary to the other grain boundary, the longest line is regarded as the long axis. Then, among the lines connecting the grain boundary of the long axis into two halves and the other grain boundary, the shortest distance is regarded as the short axis.

As for the Borne iron, after etching with a nitric acid etching solution, the area of 25 μm × 20 μm is observed by SEM at a position of 1/4 of the plate thickness to reach four observation fields, and the percentage of the layered structure that can be seen is regarded as a wave. The area ratio of iron. In the same manner, in the same manner, the corroded iron is etched with a nitric acid etching solution, and then used at a plate thickness of 1/4. The area of 25 μm × 20 μm was observed by SEM to reach four or more observation fields, and the ratio of the major axis to the minor axis was 2 or more, and it was confirmed by SEM of 5000 times that the structure of the ferritic iron was regarded as toughened iron.

The measurement results of the area ratio of each tissue are shown in Table 4 and Table 5.

[Table 4]

[table 5]

The mechanical properties of the obtained steel sheets were measured. From the test material which has been subjected to the heat treatment, a tensile test piece No. 5 conforming to the Japanese Industrial Standard JIS is adopted, that is, the direction in which the stretching is performed is the direction perpendicular to the rolling direction of the test material. Stretch the test piece and proceed Measurement: drop strength (YS), tensile strength (TS), uniform elongation (uEL), and total elongation (EL). The difference between total elongation and uniform elongation is considered to be local extensibility (lEL). The measured mechanical properties are shown in Tables 6 and 7.

All the items are in accordance with the test No. 1~5, 8, 16~35, 40~42, 48~50, 52~54 and 57~59 of the present invention, TS is 1180 MPa or more, and TS×lEL is 6000 MPa% or more. Further, YS × uEL is 10000 MPa% or more, and as a result, high strength and excellent moldability are exhibited.

On the other hand, in Test No. 6, since the annealing temperature in the first annealing step is low, the low-temperature tempering of the field has little iron and low strength. TS × lEL is also very low. In Test No. 7, since the annealing time in the first annealing step was short, the low-temperature tempering of the granulated iron was small, and similarly to the test No. 6, the strength was low and the TS × lEL was also low.

In Test No. 9, the stop temperature of the second cooling step was as low as 20 °C. After the cooling, although the second annealing step is performed, this is the same heat treatment condition as the conventional method described in Non-Patent Document 1. In Test No. 9, the low temperature tempered 麻田散铁 was only 2.0%, which was lower than the range of the present invention, so TS became low. The same is true for Test No. 45.

In Test No. 10, the cooling stop temperature in the second cooling step was as high as 400 °C. As a result, no metamorphosis occurred and the metal structure was Worth Iron. In the subsequent heating process, only a small amount of ferrite is formed, but the amount is too small, so C and Mn are not concentrated in the Vostian iron. In the third cooling step and the tempering step, a plurality of low-temperature tempered granulated loose irons are formed, and the metal structure having little residual γ is formed. Therefore, both YS×uEL and TS×tEL become low. The same is true for Test No. 51.

In Test No. 11, the maximum annealing temperature in the second annealing step was as high as 730 ° C. Therefore, the metal structure was a single-phase structure of Worthite iron, and C and Mn were not concentrated in the Vostian iron, and it became an unstable structure. Therefore, in the third cooling step and the tempering step, a lot of tempered granulated iron is generated, and the area ratio of residual γ is reduced. As a result, TS × lEL and YS × uEL become low.

In Test No. 12, since the maximum heating temperature in the second annealing step was as low as 530 ° C, precipitation of ferritic iron and wave-forming iron were changed, and the area ratio of residual γ was greatly reduced. The result, TS × lEL It becomes lower with YS×uEL.

In Test No. 13, since the annealing time in the second annealing step was only a short period of 60 seconds, there was not a sufficient time for the C and Mn to be concentrated in the Vostian iron, and the Worthite iron became unstable and the area of the residual γ was The rate is getting lower. As a result, TS × lEL and YS × uEL become low.

In Test No. 14, since the temperature in the tempering step was as low as 130 ° C, the tempering of the granulated iron produced in the third cooling step did not proceed, and the amount of fresh granulated iron in the metal structure increased. As a result, YS and YS×uEL become low. The same applies to Test Nos. 43 and 44.

In Test No. 15, the temperature in the tempering step was 600 ° C, which was higher than the range of the present invention. Therefore, the precipitation of ferritic iron was observed, and the area ratio of residual γ was lowered, and the ferritic iron was also formed to cause low temperature tempering. The area ratio of iron becomes low. As a result, YS, TS, and TS × lEL are all low.

In Test No. 46, since the average heating rate in the second annealing step was as low as 3 ° C / sec, the area ratio of residual γ was low. As a result, TS × lEL and YS × uEL become low.

In Test No. 47, since the holding time in the tempering step was long, the area ratio of residual γ was low. As a result, TS × lEL and YS × uEL become low.

In Test No. 55, since the processing conditions in the second cooling step were unsuitable, the area ratio of the high-temperature tempered granulated iron and the low-temperature tempered granulated iron was low. As a result, both TS × lEL and YS × uEL become low.

Test No. 56 is because the average cooling rate in the third cooling step is less than 2 ° C / sec, so that ferrite iron is precipitated, YR and YS×uEL becomes lower.

Test No. 36 is because the C content is lower than the range of the present invention, and the area ratio of residual γ falls outside the range of the present invention. As a result, the uniform elongation is lowered, and YS×uEL is lowered. Although the strength is also lowered, the decrease in strength is presumed to be caused by the softening of the tempered granules in the tempered granules due to the decrease in the C content.

Test No. 37, in which the C content was higher than the range of the present invention, a large amount of ferrite was present in the metal structure, and a metal structure in which many ferritic irons were observed was observed. As a result, the local ductility is greatly reduced, and TS × lEL becomes low. In addition, it broke quickly, so not only is the consistent extension low, but YS×uEL is also very low.

In Test No. 38, the Mn content was lower than the range of the present invention, and the area ratio of residual γ fell outside the range of the present invention. Therefore, the uniform elongation is lowered, and YS×uEL is also lowered. In Test No. 39, the Mn content was higher than the range of the present invention, and the Worthite iron was too stable to obtain a sufficient amount of tempered 麻田散铁, and both YS and TS became low.

In addition, in order to eliminate the influence of the alloy composition and to understand the relationship between the metal structure and the mechanical properties, the metal structure and the steel type E manufactured by the plurality of manufacturing conditions in the foregoing embodiments are used, and the metal structure thereof is The diagram of the relationship between mechanical properties is shown in Figures 1 to 6. It can be seen from Fig. 1 to Fig. 6 that by controlling the area ratio of the low temperature tempered granulated loose iron to 15.0~60.0%, the area ratio of the residual Worth iron is controlled at 10.0~55.0%, and the high temperature is returned. The area ratio of the scattered iron in the fire Ma Tian is controlled at 30.0~75.0%, and the area ratio of the fresh Ma Tian loose iron is controlled. Excellent mechanical properties are obtained in the range of 0 to 10.0%.

[Industrial availability]

According to the present invention, a high-strength steel sheet having a tensile strength of up to 1180 MPa or more and having high uniform elongation and local extensibility can be obtained.

Claims (12)

A high-strength steel sheet having a chemical composition of C: 0.10 to 0.24%, Mn: 3.50 to 12.00%, Si: 0.005 to 5.00%, Al: 0.005 to 5.00%, P: 0.15% or less, and S. : 0.030% or less, N: 0.020% or less, O: 0.010% or less, Cr: 0 to 5.00%, Mo: 0 to 5.00%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Nb: 0 to 0.50 %, Ti: 0~0.50%, W: 0~0.50%, B: 0~0.010%, Ca: 0~0.05%, Mg: 0~0.05%, Zr: 0~0.05%, REM: 0~0.05% , Sb: 0~0.50%, Sn: 0~0.50%, As: 0~0.05%, V: 0~2.0%, the rest: Fe and impurities, the metal structure at the position of 1/4 plate thickness, including the residual Worthite iron: 10.0-55.0%, High temperature tempering Ma Tian loose iron: 30.0~75.0%, low temperature tempering Ma Tian loose iron: 15.0~60.0%, the rest is fresh Ma Tian loose iron: 0~10.0%, Bora iron: 0~5.0%, toughened iron: 0~5.0%. In the high-strength steel sheet according to claim 1, in the metal structure, the total area ratio of the fresh granulated iron, the ferritic iron, and the toughened iron is 0 to 5.0% in terms of area%. The high-strength steel sheet according to claim 1 or claim 2, wherein the area ratio of the ferrite and the toughened iron in the metal structure is 0%. The high-strength steel sheet according to claim 1 or claim 2, wherein the steel sheet has a tensile strength of 1180 MPa or more and a sheet thickness of 0.8 to 3.2 mm. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition component is C: 0.13 to 0.21% by mass%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical component is Mn: 4.0 to 7.0% by mass%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition component is, by mass%, Cr: 0 to 1.50%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition is in mass%, and Mo: 0 to 1.00%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition component is Ni: 0 to 1.50% by mass%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition component is Cu: 0 to 1.50% by mass%. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition is in mass%, B: 0 to 0.003%. The high-strength steel sheet according to claim 1 or 2, comprising a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or a Zn—Ni alloy plating layer on the surface of the steel sheet.